Tyrian Purple

Fulgrim is referred to as the Phoenician.  His legion is known as the Emperor's Children and they wear purple.  These are not coincidental choices.

The Phoenicians were an ancient maritime civilization situated on the modern day coast line of Lebanon, Syria and Israel.  It was an enterprising trading culture that spread across the Mediterranean from 1500 BC to 300 BC.  The Phoenician culture helped spread the alphabet used by the modern Western world today.

One of the most prized goods that the Phoenicians traded was a dye called Tyrian purple.  It is a secretion produced by several species of predatory sea snails in the family Muricidae, rock snails originally known by the name Murex.  

The Roman mythographer Julius Pollux, writing in the 2nd century AD, asserted (Onomasticon I, 45–49) that the purple dye was first discovered by Heracles, or rather, by his dog, whose mouth was stained purple from chewing on snails along the coast of the Levant.

The reason that this dye was so prized was that it would not fade over time.   It is believed that the intensity of the purple hue improved rather than faded as the dyed cloth aged. 

The Romans valued the color so much, that it became associated with the Emperor.  A child born to a reigning emperor was said to be porphyrogenitos, "born in the purple".

Bringing the motif full circle is the symbol of the Aquila.  The eagle was the symbol of Roman emperors and is also the symbol of the Emperor of Mankind.  The Emperor's children therefore fitting bear the Aquila and the eagle's wing, alone of the legions.  

The phoenix is a mythical eagle like bird from Greek and Roman mythology, that rose from its ashes similar to how the Emperor's Children rose from the danger to their gene seed in their earliest years.  The phoenix was sometimes associated with Phoenicia in ancient times, and the term of someone from Phoenicia is a Phoenician.  La Fenice was also originally a famous opera house in Italy, which meant "The Phoenix".